Various types of laser power meters are known. As described in an article entitled "Calorimetric Measurements of Laser Energy and Power" in J. Phys. E., Vol. 6, February 1973, pages 105-114, calorimeters used for laser measurements may be generally classified as being of either the "isoperibol" or of the "conduction" type. The isoperibol calorimeter is conceptually simple in design since in principle, a single measurement of the temperature rise of the thermally isolated beam absorber is all that is required to define the beam energy which, when divided by the exposure time, gives the beam power.
One of the major disadvantages of the isoperibol calorimeter laser power meter is the requirement to accurately define the exposure time. Conventionally, in using the simplest type of isoperibol calorimeters, such as those commercially available from Optical Engineering Inc. of Santa Rosa, Calif., U.S.A., an operator is expected to time the insertion of the absorber probe into a laser beam path, using a watch. Such a technique introduces inherent inaccuracies into the measurement and makes measurement accuracy very dependent on operator skill.
In an attempt to overcome this problem, isoperibol calorimeters having electronically timed shutters have been described in the literature. Reference is made in this connection to "Calorimetric Measurements of Optical Power from Pulsed Lasers" in IEEE Trans. on Instr. and Meas., Vol. IM-21, November 1972, pages 430-433. This solution is not entirely satisfactory, since the addition of a shutter makes the calorimeter more complicated and expensive and thus commercially unviable for many applications.
U.S. Pat. No. 3,459,945 describes a laser calorimeter including a pyroelectric detector having a timed mechanical shutter to measure continuous wave beams. U.S. Pat. No. 3,622,245 describes a double plate calorimeter wherein an electronic timer is used to supply current to a calibration heater coil for a defined time, in order to define a given electrical calibration input energy.